This invention relates to novel methods for fabricating multiple layer polymeric sheet materials. It also relates to the novel apparatus used in practicing the novel methods herein, as well as to the sheet materials made therewith, and the packages made with those sheet materials.
Referring especially to the problem addressed in inventing the invention herein, vinylidene chloride copolymers (VDC) are preferred for their barrier properties to transmission of moisture vapor and oxygen.
It is commonly known that the extrusion of VDC copolymers is accompanied by a plurality of problems relating to the sensitivity of VDC copolymer to the combination of high temperature in contact with hot metal, and the time for which this high temperature contact must be maintained in the extrusion process.
So the extrusion of VDC copolymers to produce sheet materials has conventionally been accomplished by keeping the extrusion apparatus as close as possible to the die, or die feedblock, in order to minimize the time during which the VDC copolymer is exposed to the hot metal surfaces of the processing equipment. Accordingly the positioning of the VDC extruder as close as possible to the die or die feedback has more-or-less been required by this sensitivity of the VDC copolymer to the length of time during which it is exposed to the high temperature metal surfaces of the processing equipment.
Further, it is known to coextrude layers of, for example, polyethylene on either side of a VDC copolymer layer and then strip off the polyethylene layers to thus fabricate single layer VDC copolymer film. Such process, of course, provides a degree of protection for the VDC copolymer during the extrusion processing, by interposing the polyethylene between the hot metal surfaces of the die and the VDC copolymer.
It is also known to coextrude layers of other polymers with VDC copolymers, for example ethylene vinyl acetate copolymer (EVA). However, as additional layers are added to the coextrusion, additional space is required near the die to accommodate the additional equipments, such as extruders, mixers, transfer piping, die adapters, and the like. With anything more complex than fabrication of a simple 3-layer sheet, the area around the die can become quite congested. In arranging an equipment layout, and with all the polymer melt streams converging on the die, the space immediately adjacent the die is quite valuable. Thus the more versatility the layout designer has in arranging the space layout, the greater the opportunity for achieving efficiency in that layout.
Yet for coextrusions containing VDC copolymer, the VDC extruder has conventionally been positioned very close to the die equipment, as indicated above thus precluding the layout designer from choosing to use that space for anything else.
Thus it is an object of this invention to provide a method for fabricating multiple layer sheet material containing heat sensitive polymers while preserving the choice of where to locate the extruder which processes the sensitive polymer.
It is another object to provide novel apparatus for so fabricating such sheet materials.
It is still another object to provide such novel apparatus and methods at economical cost, and wherein the novel equipment is readily fabricated at economical cost.
It is yet another object to provide novel combinations of equipment to carry out the methods of the invention, in fabrication of novel sheet materials, and packages made therewith.